1. Technical Field
This disclosure relates generally to the manufacture of semiconductor devices having a multilayer interconnection structure. More specifically, this disclosure relates to novel methods of making layered tungsten plugs in such devices.
2. Background of Related Art
In the manufacture of a conventional tungsten plug or stud, a substrate 101 has a contact hole 110 formed therein as shown in FIG. 1A. A titanium layer 112 and a titanium nitride layer 114 are sequentially deposited on the substrate. Near the top edge 115 of contact hole 110, layers 112 and 114 are normally quite thin. Tungsten is then deposited, for example via chemical vapor deposition (CVD), to fill contact hole 110. Depending on the position of the substrate within the device, the structure shown in FIG. 1A can be used to produce a tungsten plug at the window level or a tungsten plug at the via level.
Tungsten is normally deposited in a single step during formation of a conventional tungsten plug. The stress in the resulting conventional tungsten film is high, usually &gt;E10 tensile. Such high stress may result in peeling of the tungsten film and the TiN layer on which the tungsten is deposited. Alternatively, ripples and/or cracks may form in the TiN layer, making the device subject to chemical attack during subsequent manufacturing steps and reducing the long term reliability of the device.
Additionally, tungsten deposition by CVD involves the use of tungsten hexafluoride, and subjects the coated substrate to fluorine and hydrofluoric acid. Diffusion of fluorine gas through pinhole defects in titanium nitride layer 114 causes excessive tungsten growth at the location of the defect.
Specifically, as shown in FIG. 1B, rupture in titanium nitride layer 114 may occur, particularly at the top edge 115 of contact hole 110. Where such ruptures occur, volcanoes 125 are formed which are areas of thick tungsten growth compared to tungsten layer produced within the contact hole 110 and the portion 122 of the tungsten layer produced on the surface of the substrate 101. Hole 110 may not be completely filled with tungsten if growth is sufficiently rapid to cover the opening of the contact hole and thereby partially or completely block any further tungsten deposition within the hole. However, even where the contact hole is completely filled by a tungsten plug 130, a large tungsten growth 126 is formed, the thickness of which far exceeds the thickness of the portion 122 of the tungsten layer on the surface of substrate 101, as shown in FIG. 1C. Ultimately, the device may have to be discarded due to the presence of the volcanoes. Where it is possible to salvage the device, additional steps must be taken to remove growth 126 from the surface of the device, adding to the cost of producing the device.
It would be desirable to provide a tungsten plug by a method which deposits tungsten with a controlled microstructure to reduce stress within the deposited tungsten. It also would be desirable to provide a tungsten plug by a method which does not produce volcanoes or undesired tungsten growths on the surface of the device.